Sound reproducer



March 21, 1933. A. N. GOLDSMITH ET AL 13023 SOUND. REPRODUCER Filed Nov.5, 1928 SOURCE OF MODULATING CURRENT sourzcE 0F mew FREQUENGYE ,1Remus-HQ, 500ml) WAVES mun MODULATOR DETEOTCR 7 /-/20 20 19 a 1/ j /M'16 Patented Mar. 21, 1933 i UNITED STATES PATENT OFFICE ALFRED 1v.aonnsm'rn Ann mvme' wour, or miw YoRx, n. Y., ASSIGNbRS 'ro RADIOCORPORATION or AMERICA, A CORPORATION 01* D LAWARE SOUND REPRODUGERApplication fl led November 5, 1828. enial No. 317,149.

This invention relates to sound reproducers such as loudspeakers, whichare adapted to reproduce sound waves without employing an electricallyor mechanically vibrated diaphragm, or which, if desired, will reproducesound waves by pressure changes without the use of a diaphragm. Morepar? ticularly this invention relates to sound reproducers in whichmeans are provided for obtaining a modulated high frequency sound waveand in which additional means are provided for detecting the modulatedhigh frequency sound wave to obtain audible frequency pulsationscorresponding to its modulations.

V I It has been suggested that sound could be reproduced by modulating asource of high.

or inaudible frequency pulsations to obtain a modulated high frequencysound wave and by radiating this wave into the open air. Such anarrangement, however, will not operate due to the fact that the humanear is not sensitive to high or inaudible frequency sound waves eventhough the sound waves are modulated at an audible frequency. We

propose to overcome this inoperativeness by employing what may be termedan acoustic detector. The function of such a device is to detect themodulated high frequency sound waves and to radiate intothe open air anaudible frequency sound wave corresponding to the modulation or envelopeof the modulated wave. The modulated high frequency sound wave can belikened to the modulated high frequency carrier Wave of radio telephonyand it can be detected in thesame manner as a modulated high frequencycarrier wave to provide anaudible frequency wave.

It is an object of this invention to provide a sound reproducerconsisting of means-for producing a modulated high frequency sound waveand means for detecting the sound wave to obtain an audible frequencywave.

A further object of this invention is to Figure 3is an end view of theacoustic detector illustrated in Figure 2;

Figure 4 is a section on line 44 of Figure 2;

of the acoustic detector employed in Figure Figure 5 is a plan view of amodified form of the acoustic detector;

Figure 6 is an end view of the modification illustrated in Figure 5;

Figure 7 is a section on line 7-7 of Figure 5; and

vFigure 8 is a, diagrammatic representation of a Venturi tube on whichthe theory of the operation of our acoustic detector is based.

Referring more particularly to the drawing 11 represents a radioreceiver or a microphone or any similar source of energy for modulatingthe cdnstant amplitude high frequency sound Waves set up by theapparatus diagrammatically illustrated at 12. The source of highfrequency sound waves illushated by reference character 12 may be amechanically vibrated member, an electrical-1y. vibrated crystal, aCorona discharge meinber, or any other means for creating high frequencypulsations consisting of condensations and rarefactions. The modulatingcurrent from the member 11 may be at a radio frequency such as themodulated carrier wave used in radio telephony, or it may be atan audiofrequency such as a detected carrier wave. The modulating current may beapplied directly to the source at 12 or it may mechanically orelectrically modu-.

modulated radio frequency currents thereto.

The modulated high frequency energy may also be obtained from a Coronadischarge which is energized with modulated radio frequency energy or itmay be obtained from a talking arc energized in the same manner I asshown by the patent to Bothe No. 1,567,230.

The beam of modulated high frequency pulsations is indicated at 13 andthe acoustic detector for rendering the high frequency pulsationsaudible, is indicated at 14. The energy from the source 12 due to its hih frequency and short wave length, can e radiated in the form of avbeamthat is'adapted to be directed in-any desired direction. This beam ofenergy is directed toward the detector 14 which may be located adjacentthe source 12 or which may be positioned some distance from the source.In the latter'case the decrease in the'energy received by the detectoris hardly noticeable due to the fact that the energy can be transmittedin a beam without diffusing or greatly decreasing in strength.

Various types of acoustic detectors can be employed for obtaining anaudible fre-.

quency wave from the modulated inaudible frequency wave just as variouscircuits can be used for detecting modulated carrier waves in radiotelephony. An approved form of an acoustic detector is illustrated inFigures 2, 3 and 4:- and a-modified form is illustrated in Figures 5, 6and 7. Referr ihg to the device illustrated by Figures 2, 3 and 4, thedetector consists of a member 15 forming a plurality of sound waveopenings or passage-ways 16 which are providedwith constrictions 17, ina manner similar to a Venturi tube. The constricted portions 17 of thesound wave openings or passage-ways 16 are connected by vents 18 withthe atmosphere. The pulsations from which the audible frequency soundwave is obtained are set up in these vents 18 by the pressure variationstaking place in these vents as th e result of the velocity variations ofthe a1r particles which are given motion by the sound waves passingthrough the passage-ways 16.

In order for the modulated high frequency sound wave to retain itsidentity and not be destroyed by interference it is advisable that theradial dimensions of the passage-ways be limited to a value not muchlarger than the wave length being transmitted. As the high frequencyenergy has a relatively short wave length it is usually necessary toemploy a plurality of openings or passage-ways 16 arranged alongside ofeach. other in order to obtain suflicient energy for the reproduction ofthe sound.

The modification illustrated by Figures 5, 6 and 7, operates in the samemanner as the detector illustrated by Figures 2, 3 and 4. In thismodification, however, a diaphragm member 19 is provided and the vent 18is modified to'form an opening 20 which will function to collect andamplify the sound waves from the diaphragm 19 in the same manner as thehorn of an ordinary loudspeaker.

The diaphragm is made of any suitable material either metallic ornon-metallic in nature and it may be clamped at its edges or suspendedby suitable flexible suspending means. A preferable method is to suspendthe diaphragm by means of a strip of velvet fastened between the edgeof, the diaphragm and the supporting member. Bymaking the restoringforceof the suspending means sufficiently low, the naturalperiod of thevibrating system is obtained at a frequency belowed by the pressurechanges resulting from the .Venturi effect.

It is usually preferable to construct the acoustic detector 14 from amaterial whose natural period of vibration does not lie within theaudible frequency range.

In explaining the theory of operation of our acoustic detector referenceis had to the Venturi tube diagrammatically represented by Figure 8. Ina Venturi tube the pressure at the constricted portion, indicated at Bin Figure 8, is not a linear function of the velocity or displacement ofair particles at that point, but contains a term which is a function ofthe square of the velocity or displacement of these particles. Thepressure is rep resented by the equation p=m'v'*l;nv (seeRaleighs'Theory on Sound, volume 2, page 13, 1926 Edition) where m andnare constants and 'v is the velocity of t airiparticles. Strictlyspeaking, this equation is true for any sound wave but ordinarily theconstant 92 is so small that its term is negligible compared with the mterm. However, when the sound wave passage-way is constricted as in termW0 produces a pressure whichwill affect the human ear. As the pressureis sufficiently great to affect the human ear and as the pressuredepends upon the square of the velocity, the ear will be able to respondto the envelope of the pressure impulses. This theory may be comparedwith the theory of detection by means of vacuum tubes.

In other Words as a sound wave, or a wave corresponding to a sound wavebut having a frequency above audibility, moves through the air it setsthe air particles in motion. The air particles move forward and backwardthrough a path forming a closed cycle. The forward motion takes place,say, with the condensation impulse and the rearward motion with thefollowing rarefaction impulse. The velocity of motion and the length ofthe path or the duration of the motion, depend upon the frequency andamplitude of the sound wave. Thus, at A and B the air particles will bemoving forward and backward at velocities proportional to the pressureand rarefa ction impulses of the sound wave and having a definiterelation to each other.' The ratio between the velocity at A and thevelocity at B being approximately inversely proportional to therespective areas.

Referring again to Figure 8, as the pres sure at D varies proportionallyto mv+nv the direction of motion of the air particles can bedisregarded. That is, the pressure variation at D has a part which isthe same, provided the velocity is the same, regardless of whether thedirection of motion is from A to C or from C to This, in effect, is thesame as if the impulses at B were all in the same direction, that is, asif the condensation impulses and rarefaction impulses were rectified soas to consist Wholly of con--- densation impulses or rarefactionimpulses. As these impulses contain the same modulation or envelope asthe original beam of high frequency energy, the ear drum can respond tothe envelope of the impulses just as the telephone diaphragm can respondto the envelope of the detected carrier wave pulsations. Thus, anaudible frequency sound wave is obtained at D.

The sound waves from the vents 18 or ffom the diaphragm 19 may becollected in amplifying horns or directed to some particular point asdesired. This is not necessary, however,- as the sound waves emanatingfrom these vents usually have sufficient amplitude so that amplifyinghorns are not required.

In the appended claims the term constricted passageway is intended todescribe a passageway having a constricted portion.

While we have illustrated only two types of acoustic detectors it is tobe understood that various other devices may be emploved for thedetection of modulating high frequency sound waves and we do not desireto be limited by the arrangement shown but only by the scope of theappended claims.

We claim: 1. An acoustic system comprising a source of modulatedsuper-audible frequency sound waves and an acoustic detector forobtaining an audible frequency sound wave therefrom.

2. An acoustic system comprising a source of modulated super-audiblefrequency sound I .waves, and means for obtaining audible sound wavesfrom said modulated super-audible frequency sound waves.

sound wave, means for modulating said su-.

per-audible frequency wave, and means for obtaining an audible frequencywave from the modulated super-audible frequency wave.

6. An acoustic system comprising means for setting air particles intomotion at a modulated super-audible frequency, and

means for obtaining an audible sound wave' from the motion of said airparticles.

7. An acoustic system comprising means for providing a beam of modulatedhigh frequency sound waves, and means independ-. ent of said firstmentioned means and positioned at a distance from said first mentionedmeans for detecting said beam of high fre' quency sound waves to obtalnan audlble frequency sound wave therefrom.

8. An acoustic system comprising means for radiating a modulatedsuper-audible frequency sound wave, and a member provided withaconstricted passage-way for detecting said sound wave to obtain anaudible sound wave therefrom. I

9. An acoustic device of the type adapted to respond to the envelope of'a modulated sound wave comprising a member provided with a constrictedpassage-way so small that non-linear effects take place in the soundwave. I v

10. An acoustic device of the type adapted to respond to the envelope ofa modulated sound wave comprising a member provided with a constrictedpassage-way sosmgll that non-linear effects take place .in the soundwave, and means cooperating with the constricted portion of saidpassage-way for radiating audible frequency sound pulsations.

11. An acoustic device of the type adapted to respond to the envelope ofa modulated sound Wave comprising a sound wave passage-way having aconstricted portion so small that non-linear effects take place in thesound-wave and a vent leading from said constricted portion forobtaining audible he I quency sound waves therefrom. s

' I 12. An acoustic device of the type adapted to respond'to theenvelopeof a modulated sound wave comprising a member provided witha'plurality of sound wave passage-ways,

each of said passage-ways havin Y a constrict ed portion so small thatnon-linear efiects take place in the sound fwave,and avent leading fromthe constricted portion of eachof said passage-ways.

'13. An acoustic detector ,comprising :a member contalnmg a soundwave-passage- I way so smallthat non-linear effects take place in thesound wave, a constricted portionin said passage-way, and a vent leadingfrom said constricted portion, the diameter of said soundwavepassage-way being'not much larger than the wave length of the sound wavebeing detected.

14. An acoustic detector comprising" a member provided with intersectingpassageways one of which is constricted to such an .extent that velocityvariations in opposite directions are converted into pressure variations1n a singledirection. I

15. An acoustic device of the type adapted torespond to the envelope ofa modulated sound wave comprising 'amem'ber provided" V ble frequencysound wave.

' one of a plurality of intersecting passage- 1 17. An acoustic deviceof the type adapted to respond to the envelo of a modulated sound wavecomprising awenturi tube, havin a portion so constricted that nob-lineare ects take place in the sound wave.

18. The method of detecting modulated high frequency sound waves whichconsists in passing the sound waves through a passageway having theinternal form of a Venturi tube.

p 19. The method of detectin modulated high frequency sound waves w ichconsists in passing said sound waves through a constricted passageway.

20. The method of detectin modulated high frequenc sound waves w ichconsists in passing sai sound waves through at least ways at least oneof which passageways has a constricted portion.

21. The'method of detectin modulated high frequency sound waves w ichconsists in passin said sound waves through a pas .sageway aving aconstricted portion, which 22. The method of detectin modulated highfrequenc sound waves w ch consists in passin sai :sound: waves through apassageway tially at right angles to the direction of the passageway'havingthe constricted portion.

modulated 23. The method of detectin high frequenc sound waves w ichconsists in passing sai sound waves through at least one oftwoeintersecting passageways which are, arranged so as toconvertvelocity variations in opposite directions into pressurevariations in a single. direction.

24. The'method of detecting modulated high frequency sound waves whichconsists in passing said waves through a passageway having a constrictedportion and having a avlnga constricted portion, the constricted portionbeing in communication with a second passageway extendin substanventincommunication .with said restricted portion.

L25. Themethod of detectin .modulated high frequency sound wavesw ichconsists,

indistortingthe sound wave so as to obtain a pressure :d1

constricte passageway.

26. An acoustic detector comprising a member provided with a constrictedpassagewa and. a movable body adjacent the constricted portion of saidpassageway and op eratively associatedtherewith whereby preswherebypressure variations in one of said assageways will cause movement ofsaid ody and thereby produce pressure variations'in the otherpassageway.

28. An acoustic detector comprising a *member provided with aconstricted passageway, a secondpassageway in said member terminatingadjacent the constricted portion 'of the first passageway, said secondpassageway fiaring in cross section from its junction with the firstpassageway.

. 29. An acoustic detector comprising a member provided with aconstricted passageway, a second passageway in said member terminatingadjacent the constricted portion of the first passageway, and a movablebody arranged at the termination of said second passageway tosubstantially shut off communication between said second passageway andthe constricted passageway.

erential varying in accordance with the modulation of said modulatedwave, I by passin said modulated wave through a 30. A systemforcommunicating intelligence comprising means providing a source oflntelhgence 1n the form of vibrations, a

